I've had no period in 6 months after Tubal Ligation!?

I had my second and last baby back in november. Right after I delivered I had my Tubal Ligation done. I was feeding at first, but my son refused to gain weight no matter how much I fed him, I had plenty of milk and no latching problems, he just wasnt gaining weight. So I switched him to the bottle and within two weeks I got my period back, this was back in Jan. it was very normal 4 days, and thought everything was fine. Then the next month I spotted only lightly for two days and ever since havent had anything remotely close to a period. I do have severe cramps when it is due, but still nothing! It has been 6 months plus now, and I am worried. I took four home preg. tests, over a length of time, and all were negative, along with three from the doctors office, they were neg too. The doctor told me everything looks normal on my bloodwork. So why are my periods absent? Does anyone have a clue what could be going on? I am not a thin woman, im 24 and normally regular as clock work...?